In a sheet metal stretch forming process, a set of forming tools normally consists of four major pieces. They are a die, a punch, an upper retainer ring, and a lower retainer ring. The retainer rings are also known as binder rings. The upper and lower retainer rings are used in holding a sheet metal workpiece in place such that a die and a punch could work jointly to deform the workpiece into a desired shape. In a usual setup, the upper and lower retainer rings and the punch are movable while the die is stationary.
Traditionally, the retainer rings are machined of high strength tool steel. While the strength and durability of tool steel are adequate for sheet metal forming use, extensive machining is required to obtain the desired shape. As a consequence, it is very expensive to make a retainer ring with tool steel material. This machining cost becomes even more prohibitive when prototype low volume tools are made. In a prototype tool, many changes from the original design are made before a final design is reached. In the case of low volume tools such as that used in a limited production of a concept car, the tools must also be made inexpensively.
To save time and expenses involved in machining retainer rings in tool steel, attempts have been made to make retainer rings by casting a polymeric material. High strength polymeric materials such as thermoset epoxies have been used for this purpose. The major benefit realized in using cast epoxy retainer rings is that they may be cast to size saving the expenses of machining. However, while epoxy materials have good mechanical properties, we found that epoxies used alone without other reinforcement produced retainer rings that are easily failed after a few forming cycles. We have observed that most of the failures occurred at the rim portion, i.e., material immediately adjacent to the brim of the opening of the retainer ring. This is because in a sheet metal stretch forming process, the rim portion of the retainer ring which grips the edges of the metal sheet preventing it from slipping into the die cavity is subjected to very large radially inward stresses. As a consequence, chunks of epoxy material at the rim portion frequently broke away from the body portion of the retainer ring.
It is therefore an object of the present invention to provide a mechanically reinforced plastic retainer ring that can be used in a sheet metal stretch forming process without failure.
It is another object of the present invention to provide a metal bracket reinforced plastic retainer ring to be used in a sheet metal stretch forming process which can be made at low cost.